1. Field of the Invention
This invention relates to metal-halogen cells having an aqueous solution of a metal halide as the electrolyte. In particular, the present invention relates to improved cells and batteries employing a zinc or cadmium anode, a bromine cathode and an aqueous metal bromide electrolyte in which the metal of the metal bromide is the same as the metal of the anode.
2. The Prior Art
Cells for the production of electricity having two electrodes, one with a high positive oxidizing potential, the anode, and one with a strong negative or reducing potential, and cathode, having long been known. Typical of such type cells is metal halogen cells in which the anode material most commonly employed is zinc, and the most commonly employed cathodic halogen is bromine. Among the advantages of said cells is their extremely high theoretical high energy density. For example, a zinc-bromine cell has a theoretical energy density of 200Wh/lb (i.e., watt hours per pound) and an electrical potential of about 1.85 volts per cell.
In such a cell the surface of the metal anode, for example, zinc, oxidizes thereby undergoing a positive increase in valence. As a result thereof, zinc atoms are converted to zinc ions which enter the electrolyte according to the equation: EQU Zn .fwdarw. Zn.sup.++ +2e
The chemical reaction occurring at the cathode is expressed by the following equation: EQU Br.sub.2 + 2e .fwdarw. 2Br.sup.-
Thus, the overall chemical reaction can be written as follows: EQU Zn + Br.sub.2 .div. Zn.sup.++ + 2Br.sup.-
The arrow to the right indicates the direction of the chemical reaction occurring during cell discharge and the arrow to the left indicates the chemical reaction occurring during charging of the cell.
The electrochemical cells of the foregoing type are known to suffer from a number of disadvantages. Most of these disadvantages are associated with side reactions which may occur in such cells. For example, during the charging process free bromine is produced in the cell. This free bromine is available for a chemical reaction with the metal anode thereby resulting in an auto-discharge of the cell. Additionally, there is the tendency for hydrogen gas to be generated when considerable amounts of free bromine are present in the aqueous phase. It is believed that hydrogen is generated according to the following chemical reactions: EQU Br.sub.2 + H.sub.2 O .fwdarw. HBr + HBrO EQU 2hbr + Zn .fwdarw. ZnBr.sub.2 30 H.sub.2
the art is replete with efforts on the part of many inventors to overcome the above-mentioned disadvantages. In U.S. Pat. No. 2,566,114, for example, the use of tetraethyl and tetramethyl ammonium bromides for combining with bromine generated during charging of the cell is disclosed. The tetramethyl ammonium salt is added to the powdered carbon surrounding the cathode.
In U.S. Pat. No. 3,738,870 the use of a solid mixture of alkyl ammonium perchlorate and conductive materials such as graphite to form solid addition products with halogen released during charging of such cells is disclosed.
In U.S. Pat. No. 3,811,945 the use of certain alkyl ammonium perchlorates, diamine bromides and diamine perch lorates, which are capable of forming solid addition products with cathodic bromine and which are substantially insoluble in water is disclosed.
In contrast to those references which suggest forming solid addition products with bromine, U.S. Pat. No. 3,408,232, discloses the use of an organic solvent for elemental bromine in such aqueous zinc-halogen batteries; U.S. Pat. No. 3,816,177 discloses the use of a quaternary ammonium halide and a depolarizer in the electrolyte. The depolarizer is an organic complexing solvent which dissolves in water and is non-reactive toward the halogen in the cell and forms a water insoluble complex in the presence of quaternary ammonium halides.
These references and many others show a continuing effort on the part of many inventors to overcome some of the disadvantages associated with metal halogen cells of the type referred to herein. Unfortunately, the methods proposed for overcoming the aforementioned disadvantages have not adequately overcome the problems encountered in such systems. There is, consequently, a need for more effective methods for preventing loss of cell capacity in aqueous metal-halogen cells.